The invention relates to an electron beam apparatus, comprising an electron source which comprises a metal wire which is displaceable in the longitudinal direction and which is to be locally heated by an energy beam, and also comprising a beam-limiting diaphragm which is disposed near the wire and which carries a positive potential with respect to the wire.
An electron beam apparatus of this kind is known from U.S. Pat. No. 3,745,342. In an apparatus described therein, a metal wire which is heated, for example, by means of an electron beam or a laser beam, is used as a cathode for the apparatus. In order to ensure an adequate service life of the source, the metal wire is displaced in the longitudinal direction during operation.
An electron current emerging from an extremely narrow line of such a metal wire exhibits a comparatively large spread in the emission direction in the case of a wire having a circular cross-section of small dimension. Because a lens for further processing of the electron beam inherently has a spherical aberration which is comparatively large with respect to the line thickness, a beam-limiting diaphragm is required in order to form a target spot having a small dimension in at least one direction.
Due to such large directional spread, however, a diaphragm of this kind is likely to transmit an insufficient part of the beam current. In the known electron beam apparatus, therefore, in order to obtain an acceptable service life of the source, limitations must be imposed which adversely affect aspects of the target spot quality such as maximum current density and minimum spot size.